1. Field of the Invention
The present invention relates to optical transmission systems, and more particularly to a system that optically transmits a frequency-division-multiplexed signal obtained by frequency-division-multiplexing a plurality of signals.
2. Description of the Background Art
FIG. 11 is a block diagram exemplarily showing the configuration of a conventional optical transmission system for transmitting a frequency-division-multiplexed signal. As will be known from FIG. 11, this optical transmission system comprises a multiplexer 1100, an FM modulator 1101, an optical transmitter 1104, an optical receiver 1106, and an FM demodulator 1107. In the optical transmission system, an electrical transmission line 1102 connects the FM modulator 1101 and the optical transmitter 1104 to each other, and an optical transmission line 1105 connects the optical transmitter 1104 and the optical receiver 1106 to each other.
The operation of the conventional optical transmission system in the above configuration will be described below. The multiplexer 1100 frequency-division-multiplexes a plurality of signals, and outputs the resultant signal to the FM modulator 1101. The FM modulator 1101 converts the frequency-division-multiplexed signal into a frequency-modulated signal (hereinafter, referred to as xe2x80x9cFM modulated signalxe2x80x9d) having a predetermined frequency deviation through frequency modulation. After that, the FM modulator 1101 outputs the FM modulated signal to the electrical transmission line 1102. The optical transmitter 1104 receives the FM modulated signal through the electrical transmission line 1102, then converts the signal into an optical signal, and sends the optical signal to the optical transmission line 1105. The optical receiver 1106 receives the optical signal through the optical transmission line 1105, then converts the signal into an FM modulated signal which is an electrical signal, and outputs the FM modulated signal to the FM demodulator 1107. The FM demodulator 1107 demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal.
The optical transmission system in the above configuration is described in detail in xe2x80x9cOptical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systemsxe2x80x9d by K. Kikushima et al. (IOOC""95 Technical Digest, Vol. 5 PD2-7, pp.33-34), and other documents. The optical transmission system converts a frequency-division-multiplexed signal into an FM modulated signal, and then optically transmits and demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal. The optical transmission system utilizes an FM gain in the FM transmission to improve the signal-to-noise power ratio (SNR) of the demodulated signal (i.e., the frequency-division-multiplexed signal), thereby enabling high-quality signal transmission.
Thus, the above-described optical transmission system can realize high-quality multi-channel signal transmission with an optical fiber.
However, the above-described system for optically transmitting an FM modulated signal has the following specific problems due to the properties of the FM modulated signal and the nonlinearity of an optical fiber.
An FM modulation scheme increases a frequency deviation to acquire a greater FM gain, thereby enabling signal transmission of higher quality than other modulation schemes such as amplitude modulation. On the other hand, the increased frequency deviation requires a wider signal band. In addition, in the FM modulation scheme, linear distortion tends to occur under the influence of the group delay characteristic of a transmission line and the like (the characteristic that a propagation delay varies depending on a frequency). Therefore, the transmission line must be designed with particular attention. However, as a signal band becomes wider, the group delay variations in the band become more difficult to sufficiently suppress.
In a general optical modulation scheme, the optical frequency spectrum, of an optical signal is composed of a steep-shaped optical carrier component, which has narrow spectral line-width, and upper and lower sidebands, as shown in FIG. 12B. The upper and lower sidebands are geometrically similar to the frequency spectrum of a modulating signal. Therefore, if a wide-band signal like an FM modulated signal is used as a modulating signal in optical modulation, the optical frequency spectrum of the optical signal also becomes wider. The optical signal having such wide optical frequency spectrum becomes susceptible to the chromatic-dispersion of an optical fiber (the characteristic that a propagation delay varies depending on a wavelength). The affected optical signal component interacts with the optical carrier component to induce harmonic distortion in the FM modulated signal, resulting in waveform deterioration of the transmitted signal.
As is known from the above, the conventional optical transmission system has the specific problem that the quality of the transmitted signal is degraded due to the wide-band property of an FM modulated signal.
Therefore, an object of the present invention is to provide an optical transmission system capable of narrowing the bandwidth of an FM modulated signal while increasing the frequency deviation thereof to realize high-quality signal transmission. The present invention has the following features to attain the object above.
A first aspect of the present invention is directed to a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division multiplexing a plurality of signals, from a transmitting end to a receiving end. The transmission system comprises at the transmitting end, a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal, an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal, and an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in the optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to the receiving end. The transmission system also comprises at the receiving end, an optical receiver for receiving the optical-intensity-modulated signal from the optical transmitter, and converting the optical-intensity-modulated signal into an electrical signal corresponding to the FM modulated signal through photodetection based on a square-law detection characteristic to output the electrical signal as a received FM modulated signal, and an FM demodulator for demodulating the received FM modulated signal to reproduce the frequency-division-multiplexed signal.
As described above, in the first aspect, the FM modulated signal is obtained through frequency modulation using a frequency-division-multiplexed signal as an original signal. The FM modulated signal is converted into an optical-intensity-modulated signal at the transmitting end. The optical-intensity-modulated signal has an optical frequency spectrum in which upper and lower sidebands distribute geometrically similarly to the frequency spectrum of the original signal for the optical modulation and in which an optical carrier component is suppressed. Then, the optical-intensity-modulated signal is photodetected based on a square-law detection characteristic at the receiving end. At the receiving end, the optical transmission system thus obtains an FM modulated signal, having a frequency deviation twice as large as the one of the original FM modulated signal produced at the transmitting end, as a received FM modulated signal. In this manner, the optical transmission system can narrow (reduce in half) the bandwidth of the FM modulated signal at the transmitting end while securing the frequency deviation thereof large enough to acquire a sufficient FM gain in FM demodulation. As a result, it is possible to prevent the waveform of the transmitted signal from being deteriorated due to the group delay characteristic of the electrical transmission line and the chromatic-dispersion of the optical transmission line, and to realize signal transmission of good quality.
According to a second aspect, in the first aspect, the optical transmitter includes a light source for outputting an unmodulated light, and an optical modulator for modulating the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal. The optical modulator has the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and is biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the minimum.
As stated above, in the second aspect, the optical modulator used herein is an external optical modulator having the Mach-Zehnder interferometer structure. A modulating signal (an FM modulated signal) is applied to the optical modulator with respect to the xe2x80x9cvalleyxe2x80x9d where the output optical power is at the minimum in the input-voltage vs. output-optical-power characteristic (which is periodic like a sine wave) of the optical modulator. The optical modulator thus produces an optical-intensity-modulated signal whose optical carrier component is suppressed. The suppression of the optical carrier component prevents the waveform from being deteriorated by the chromatic-dispersion of the optical transmission line. In addition, the optical-intensity-modulated signal has an optical frequency spectrum in which upper and lower sidebands distribute geometrically similarly to the frequency spectrum of the original signal for the optical modulation. Therefore, after the optical-intensity-modulated signal is square-law detected at the receiving end, the frequency deviation of the FM modulated signal is doubled, thereby making it possible to realize high-quality signal transmission.
According to a third aspect, in the second aspect, the transmission system further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is xc2xdn the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
As described above, in the third aspect, the optical transmission system previously produces in the FM modulator an FM modulated signal having a frequency deviation larger enough to acquire a desired FM gain. The optical transmission system then converts the FM modulated signal into a frequency-divided FM modulated signal, and next converts the frequency-divided FM modulated signal into an optical-intensity-modulated signal for transmission. This reduces the phase noise in the FM modulated signal to be optically transmitted and FM demodulated. As a result, high-quality signal transmission can be realized.
According to a fourth aspect, in the first aspect, the optical transmitter includes a light source for outputting an unmodulated light, an optical branching circuit for branching the unmodulated light fed from the light source into first and second unmodulated lights, an optical modulator for modulating the first unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, the optical modulator having the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and being biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the maximum, and an optical combining circuit for combining the optical-intensity-modulated signal produced by the optical modulator and the second unmodulated light to cancel the optical carrier component of the optical-intensity-modulated signal with the second unmodulated light and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
As described above, in the fifth aspect, the optical-intensity-modulated signal produced by the optical modulator is combined with the second unmodulated light set in an opposite phase to the optical carrier component of the optical-intensity-modulated signal. The optical carrier component of the optical-intensity-modulated signal is thus canceled by the second unmodulated light. As a result, it is possible to produce an optical-intensity-modulated signal whose optical carrier component is suppressed.
According to a sixth aspect, in the fourth aspect, the transmission system further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is xc2xdn the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
As stated above, in the sixth aspect, as in the third aspect, the optical transmission system previously produces in the FM modulator an FM modulated signal having a frequency deviation larger enough to acquire a desired FM gain, then converts the FM modulated signal into a frequency-divided FM modulated signal, and converts the signal into an optical-intensity-modulated signal for transmission. It is therefore possible to reduce the phase noise in the FM modulated signal to be optically transmitted and FM demodulated.
According to a seventh aspect, in the first aspect, the transmission system further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is 1xc2xdn the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical transmitter includes an optical modulator for producing the optical-intensity-modulated signal through the optical modulation using the frequency-divided FM modulated signal as an original signal.
As described above, in the seventh aspect, the optical transmission system previously produces in the FM modulator an FM modulated signal having a frequency deviation larger enough to acquire a desired FM gain, then converts the FM modulated signal into a frequency-divided FM modulated signal, and next converts the signal into an optical-intensity-modulated signal for transmission. It is therefore possible to reduce the phase noise in the FM modulated signal to be optically transmitted and FM demodulated.
An eighth aspect of the present invention is directed to an transmitter for use in a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division-multiplexing a plurality of signals, from a transmitting end to a receiving end. The transmitter comprises a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal, an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal, and an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in the optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to the receiving end.
According to a ninth aspect, in the eighth aspect, the optical transmitter includes a light source for outputting an unmodulated light, and an optical modulator for modulating the unmodulated light with the FM modulated signal to produce the optical-intensity-modulated signal, the optical modulator having the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and being biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the minimum.
According to a tenth aspect, in the ninth aspect, the transmitter further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is xc2xdn the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
According to an eleventh aspect, in the eighth aspect, the optical transmitter includes a light source for outputting an unmodulated light, an optical branching circuit for branching the unmodulated light fed from the light source into first and second unmodulated lights, an optical modulator for modulating the first unmodulated light with the FM modulated signal to produce the optical intensity-modulated signal, the optical modulator having the Mach-Zehnder interferometer structure with a predetermined input-voltage vs. output-optical-power characteristic, and being biased in the input-voltage vs. output-optical-power characteristic such that the output optical power is at the maximum, and an optical combining circuit for combining the optical-intensity-modulated signal produced by the optical modulator and the second unmodulated light to cancel the optical carrier component of the optical-intensity-modulated signal with the second unmodulated light, and output the optical-intensity-modulated signal whose optical carrier component is suppressed.
According to a thirteenth aspect, in the eleventh aspect, the transmitter further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is xc2xdn the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical modulator modulates the first unmodulated light with the frequency-divided FM modulated signal to produce the optical-intensity-modulated signal.
According to a fourteenth aspect, in the eighth aspect, the transmitter further comprises a frequency-divider provided between the FM modulator and the optical transmitter for converting the FM modulated signal outputted from the FM modulator into a frequency-divided FM modulated signal whose frequency is xc2xdn the frequency of the FM modulated signal, the n being an integer of not less than 1, wherein the optical transmitter includes an optical modulator for producing the optical-intensity-modulated signal through optical modulation using the frequency-divided FM modulated signal as an original signal.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.